1. Field of the Invention
This invention relates to oligomeric siloxane compounds having liquid crystal properties.
2. Background Information
It is well known that monomeric liquid crystals consist of compounds having an elongated or rod-like structure usually with a rigid core. Such molecules, which usually contain a permanent electrical dipole and easily polarisable chemical groups, may exhibit nematic (N), chiral nematic (N,), smectic (S) and chiral smectic (S*) mesophases but on cooling to lower temperatures experience a transition to a solid crystal. This liquid crystal to solid crystal transition destroys the liquid crystalline order. Side chain polymer materials are known which exhibit similar liquid crystalline phases but at lower temperatures undergo a transition from one liquid crystal state to a viscous or glass state thereby storing the liquid crystalline order. Liquid crystal phases, or mesophases, show varying degrees of molecular ordering between the almost perfect three dimensional structure of a crystalline solid which exhibits positional and orientational order and the randomly ordered state of an isotropic fluid.
In the nematic phase (N) all positional order is lost so that the centres of mass of the molecules are arranged randomly in space. The orientational order is, however, maintained so that there is a statistical orientational ordering of the molecules parallel to their long axes. Such phases may have the direction of their alignment altered by the application of mechanical, electrical, optical or magnetic fields. The ability to switch the direction of the alignment gives rise to a display or device element that can be used, for example to display information. Liquid crystal display elements based on the nematic phase are widely used in electro-optical devices such as the displays of digital wrist watches, calculators, word processors, personal computers and the like. However, the nematic liquid crystal material presently used in these displays has problems in terms of its bistability or memory property and of its inapplicability to a high speed switching element.
In the chiral nematic (N*), or cholesteric, mesophase the molecular order is characterised by an orientational order similar to that found in nematics but in this phase the axis direction changes continuously along an axis perpendicular to the first and traces out a helical path. This mesophase requires that the mesogenic material is optically active or contains optically active additives to produce the twisted or chiral nematic mesophase. If the pitch of the helix is of the order of the wavelength of visible light then a characteristic of this N* phase is a bright selective colour reflection. Such chiral nematic mesophases are often used in thermography since slight temperature changes distort the helical pitch and this leads to a change in colour of the reflected, and therefore also, transmitted light.
In a smectic phase the molecular order is characterised by orientational order and two degrees of directional order giving rise to a lamellar structure. Within this broad phase class there are many types of smectic phases depending on whether the centres of mass of the molecules in each layer are randomly arranged (as in a S.sub.A phase) or ordered between themselves (as in a S.sub.B phase) whether the lamellar layers are correlated or whether the orientational order is tilted at some angle to the layer normal as might be the case for a S.sub.C phase. Smectic phases may be aligned in electrical, magnetic or optical fields to give devices with a memory or information storage capability. In the case of low molar mass compounds this in electrical, magnetic, mechanical or optical fields to give devices with a memory or information storage capability. In the case of low molar mass compounds this memory effect is mechanically fragile whilst in the case of polymers the memory is robust but the response time is much slower.
In a chiral smectic phase (S.sub.C *) the orientational order is normally inclined to the layer normal, as in a S.sub.C phase, but the direction of the orientation changes continuously along the axis of the layer normal thereby tracing out a helical path rather like a corkscrew. Various chiral smectic phases exist depending on the type of orientational order within the layer. Such chiral mesophases normally exhibit ferroelectric properties and it is known that a liquid crystal display element containing such a chiral mesophase, a so-called ferroelectric, is capable of high speed response, in the order of 10 microseconds, and has a memory property.
Low molar mass liquid crystals having chiral and non-chiral nematic or smectic structures are known and because of their optical and electrical properties have found many technological uses especially in the opto-electronics field. However, the known materials have some limitations on their performance which restricts their ultimate applicability.
Recently much work has gone into the study of low molar mass (LMM) liquid crystals with electro-optic properties suitable for use at ambient temperatures. Since one highly desirable property was fast electro-optic switching, and because this switching time depends on the cooperative molecular reorientation, attention was focussed on the synthesis of relatively small molecules of low mean viscosity. However, despite the wide range of materials prepared it is only quite recently that electro-optic devices have become firmly established with the discovery of the cyanobiphenyl family of compounds. At lower temperatures these compounds exhibit crystalline phases which limit their response time in the mesomorphic phase and destroy the induced order on cooling from the said mesophase to the crystalline phase. Although LMM liquid crystals have been used for storage of the induced order in, for example a smectic phase, there are a number of disadvantages as follows:
1. the stored information in the smectic phase is often easily lost by mechanical or thermal stress;
2. cooling into the inherent crystal phase destroys the induced order;
3. grey scaling which is the production of different degrees of controlled light transmission or scattering is difficult, and
4. difficulties arise in controlling the alignment on cooling from the isotropic phase since the materials generally prefer to align homeotropically, that is perpendicular to the substrate, rather than predominantly parallel in a high optical contrast scattering state.
Having regard to these disadvantages there exists a need for improvement in such materials.
It is therefore an object of the present invention to provide novel low mass liquid crystal materials having siloxane-containing structures and mixtures containing them, that may be incorporated into a wide variety of opto-optic, magneto-optic, electro-optic and mechanical or thermo-optic storage and non-storage devices.
Siloxane-containing liquid crystal polymers in which the mesogen is present as a side chain have been disclosed in U.S. Pat. No. 4,358,391 and GB 2 146 787B. Siloxane-containing liquid crystals have been disclosed in EP-A-0 322 703 which relates to a liquid crystal composition comprising a main chain-type mesomorphic polymer and a mesomorphic monomer and showing a smectic phase. EP-A-0 478 034 relates to a homogeneous electro-rheological fluid which mainly comprises a liquid crystal compound in which a plurality of liquid crystal groups are bonded to a molecular chain, or comprises a lyotropic liquid crystal comprising a solute and a solvent. The liquid crystal compound may have a siloxane molecular chain. Siloxane-containing chiral smectic liquid crystals are disclosed in JP 01144491 and JP 01268785 and nematic siloxane-containing liquid crystals are disclosed in JP 02180890.